MM-6992 Added highlighting to elasticsearch results (#8861)

* MM-6992 Added highlighting to elasticsearch results

* Added a unique type for post search matches

* Fixed Elasticsearch matches not being sent through API

einterfaces/elasticsearch.go

@@ -12,7 +12,7 @@ import (
 type ElasticsearchInterface interface {
 	Start() *model.AppError
 	IndexPost(post *model.Post, teamId string) *model.AppError
-	SearchPosts(channels *model.ChannelList, searchParams []*model.SearchParams) ([]string, *model.AppError)
+	SearchPosts(channels *model.ChannelList, searchParams []*model.SearchParams) ([]string, model.PostSearchMatches, *model.AppError)
 	DeletePost(post *model.Post) *model.AppError
 	TestConfig(cfg *model.Config) *model.AppError
 	PurgeIndexes() *model.AppError